Ink jet printing is accomplished by ejecting ink from a nozzle toward paper or other medium upon which printing is to be placed. In various printing processes ink is driven from the nozzles toward the medium in different ways.
Among the ways to perform ink jet printing are electrostatic printing which is performed by driving ink from a nozzle toward a medium with an electrostatic field. Another ink jet printing procedure employs a piezo-electric element located within the ink nozzle. Electrically-caused distortions of the piezo-electric element causes it to operate in a pump-like fashion to drive the ink through the nozzle and toward the printing medium. Still another ink jet printing procedure drives ink from a nozzle and toward the printing medium by forming an expanding vapor-phase bubble behind the nozzle. The vaporizable material that forms the bubble is called the driver and the major portion of the ink that is deposited on the printing medium and forms the visible image is called the carrier. The carrier includes a colorant which is usually a dye or pigment that is dissolved or suspended in the ink and it forms the visible image on the medium. Some driver material may be mixed in with the carrier when it is deposited on the paper, although in this state it no longer functions as a driver. The driver and the carrier are sometimes referred to as the vehicle of the ink. Solid ink jet inks are those having carriers that are solid at room temperature These various printing methods are described in Vaught, et al., U.S. Pat. No. 4,490,728; Output Hard Copy Devices, edited by Durbeck and Sherr, Academic Press 1988, see particularly chapter 13 entitled Ink Jet Printing; and EPC Application No. EP0036790.
When a solid ink jet ink having a vaporizable driver is used it is common to hold a capacity of ink at a temperature that is higher than the melting temperature of the carrier, which is higher than room temperature. That higher temperature is called the operating temperature and it is usually between about 70 degrees C. and 110 degrees C. depending on the melting temperature of the carrier. Holding ink at the operating temperature insures that the carrier is in liquid phase and diminishes the amount of heat that must be supplied to vaporize the driver.
In order to drive the ink through the nozzle it is necessary to form a vapor phase bubble from the liquid driver. The bubble is usually formed by passing the ink composition which includes the driver into contact with an electric resistance heater that is pulsed with a very short electrical pulse. This pulse rapidly raises the temperature sufficiently high to vaporize the driver in contact with the heater to well above its boiling temperature so that a great deal of superheat is added to the driver. The peak temperature of the heater used to vaporize the driver is usually between 150 and 400 degrees C. but it may be different depending on the driver. When the driver in contact with the heater is at a superheated temperature it can vaporize suddenly and explosively to provide the driving force necessary to eject ink from the nozzle. The heater normally provides heat to the ink in pulses. The driver may be vaporized with a single heat pulse or with multiple heat pulses. The heat pulses usually have a duration between one and ten microseconds. In order for a driver to be vaporized with a single heat pulse, it must have very high volatility at temperatures near its boiling temperature. It is desirable to vaporize a driver with a single heat pulse it is a more energy-efficient mode of operation than if multiple pulses are required.
Holding ink at elevated operating temperature causes a volatile driver to evaporate more quickly than when it is held at room temperature so that the composition of the ink changes constantly. When too much driver evaporates the ink becomes viscous and more difficult to eject and there is less volatile driver available to form bubbles. When ink is held at elevated operating temperature too long successful ejection of the ink through a nozzle cannot be accomplished or the ink may form a crust so that it becomes unusable.
One necessary property of a driver is that it be a solvent for the carrier. Another necessary property of a good driver is that it be very volatile at temperatures close to its boiling temperature so that it can be vaporized easily. A very desirable quality of a driver is that it evaporate slowly when it is held at its operating temperature In some respects the desirable qualities of a driver are inconsistent. Normally one has to trade off easy bubble forming ability for staying time at operating temperature, or vice versa.